The present invention relates generally to image forming systems, and specifically relates to latent image developer subsystems in such systems.
Toner imaging systems of the type where a latent charge image is developed with a pigmented toner are widespread in the office and home. Once developed with the toner, the image is transferred to a receiving member to form a printed image on a substrate, such as a sheet of paper.
Many technologies exist for forming a latent charge image, including optical image projection onto a charged photoconductive belt or drum, charging a dielectric member with an electrostatic pin array or electron beam, and charge projection from a so-called ionographic print cartridge or plasma generator. Once a latent image is formed, the latent image may be transferred to an intermediate member before development. Alternatively, the latent image may also be developed on the same member as that on which it is formed, with different system architectures having evolved to address different process priorities, such as cost, speed, preferred type of toning system or intended receiving substrate.
Regardless of the image forming system utilized, an image developer having a developer roll and toner is typically utilized to develop the latent image. The developer roll, having a supply of toner, transfers the toner to the imaging member to develop the latent image thereon. Toner conditioning and feeding to the developer roll is commonly done gravitationally, along with mechanical agitation to prevent agglomeration or lumping of toner particles. Such lumping makes it difficult to develop the image uniformly, detect the toner level, and can result in print deletions. The mechanical and electrical properties of the toner are affected by environmental moisture and compaction.
One attempt to circumvent the aforementioned problem has used a gas to convey the toner to different parts of the image developer. Specifically, a stream of rapidly moving gas is used to convey the toner from one device to the next. The stream of gas helps prevent the toner from lumping. Unfortunately, the relatively rapid speed of the fluid used to convey the toner results in toner loss to the atmosphere. The use of a conveying gas, therefore, is often accompanied by additional hardware, such as filters and covers, to attempt to recapture the toner particles lost by the conveying process; however, even if a filter or a cyclone or both can be used to collect the particles conveyed away, the toner loss is not eliminated because the collected particles are not reusable.
Because of the aforementioned problems associated with the use of a toner in an image forming system, there exists a need for a gentle means of transporting, agitating, and conditioning toner particles in an image developer so that the toner is delivered to the developer roll without excessive loss of toner, and in a state conducive to consistent development.
In the present invention, an image developer system employs a fluid (i.e., a gas or liquid) to fluidize the toner particles for conditioning and transporting the toner within the image developer system without mechanical agitation, or conveyance. When in the state of fluidization, the toner behaves like a liquid, therefore allowing liquid-like handling. The hydrostatic pressure of the fluidized toner is advantageously employed to measure and detect the toner level, and to transport the toner. The fluidization process is gentle enough to prevent substantial loss of toner particles to the atmosphere, but intense enough for proper mixing. The use of dry air as the fluidization fluid, with a dew point brought below xe2x88x9240xc2x0 F. at room temperature and atmospheric pressure, aids in toner delumping, and stabilizes the fluidization process.
A bed of toner particles may be subjected to a stream of fluid, such as air, moving at a given velocity. If the velocity of the fluid stream is made to increase, there arrives a point at which the vertical component of the drag force exerted by the fluid stream on the particles approximately cancels the gravitational force on the particles. The particles become suspended, and are said to be fluidized. As the velocity of the fluid stream increases, the pressure drop across the bed remains essentially constant. In this regime, where the pressure drop remains essentially constant, the toner particles are still fluidized. As the velocity of the fluid is increased further, however, there comes a point where the vertical component of the drag force and the gravitational force no longer cancel. The magnitude of the vertical component of the drag force exceeds the magnitude of the gravitational force, and the toner particles are carried by the fluid stream. This point signals the end of the fluidization regime, when the fluid ceases to be fluidized, and the start of the conveyance regime. A fluidized toner is used advantageously in the invention described herein.
In particular, an image developer system for providing a generally fluidized toner suitable for use in an image forming system is described herein. The image developer system includes a chamber for housing toner particles, and a fluid source for introducing fluid into the chamber at a velocity to fluidize the toner particles to yield a generally fluidized toner having substantially fluid characteristics. The velocity of the fluid introduced into the chamber may be between about 0.003 cm/s, for lightweight (0.5 g/cm3) and small (5 micrometers) toner particles, and about 8.4 cm/s, for heavy (3 g/cm3) and big (30 micrometers) toner particles. The velocity may be defined as a ratio of the volumetric flow rate of the fluid to the cross-sectional area of a fluidized bed.
The image developer system may further include a pressure distributor for distributing the fluid substantially evenly throughout a bottom of the chamber, and a level sensing subsystem for measuring a level of the toner particles in the chamber. Moreover, the fluid source may include a conditioning element to condition the fluid prior to introduction to the chamber. The chamber may also have an angled wall for promoting circulation of the generally fluidized toner therein. In addition, the image developer system may include a developer roll for attracting the fluidized toner onto a surface thereof.
The level-sensing subsystem may include a bubble tube. Specifically, the liquid-like behavior of the fluidized toner allows the use of the bubble-tube to detect the toner level in a fluidized chamber. A hollow (a few mm in diameter) tube, fixed to a wall of a chamber, and immersed in the toner, may duct a low velocity (a few cm/s) flow of the same fluidizing fluid. The static pressure at the outlet of the tube is equal to the hydrostatic pressure of a column of the fluidized toner above the outlet. A pre-set pressure switch, hermetically attached to the bubble tube, can detect the level corresponding to the pre-set pressure value of the switch, providing an electrical signal to a process controller. Many differently pre-set switches may be attached to a single bubble tube to detect many predetermined toner levels. For example, two pre-set switches can detect two levels, three pre-set switches can detect three levels, etc.
The use of a particular fluidizing fluid to fluidize the toner helps to treat or condition the toner to maintain or to modify the properties of the toner particles for effective development of images. For example, moisture in the ambient (atmospheric) air favors the formation of toner lumps and affects the stability of the electrical properties (conductivity) of toners. Fluidization may be successfully employed to keep the toner dry, thereby prevent the variation in the conductivity of the toner, resist the tendency of toner bridging and lump formation, and assist in toner de-lumping if it occurs. For example, the use of dry air as a fluidizing fluid, with a dew point brought below xe2x88x9240xc2x0 F. at atmospheric pressure, may be advantageous. The fluidization process also provides the means of transporting the toner in a controlled fashion without the use of mechanical methods which often produce undesirable effects.